Abstract

Direct measurement of pebbles, cobbles, and other large grains by using a suitable instrument equipped with a phi scale is a time-saving method. The scale is constructed by first choosing the desired phi intervals and then determining the distances in millimeters between each interval, beginning at the small end of the scale (infinity). The millimeter distances of phi intervals are found by using a phi-millimeter conversion table or from the formula millimeters = antilog 10 -phi /3.322. Direct measurement of sand-size and smaller grains can be accomplished by using a scale calculated as above in a micrometer occular. The actual size of the grain is found from the enlargement in phi units and the following: log 2 (magnification) = 3.32log 10 (magnification).

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